Piezoresistivity in conductive polymer nanocomposites occurs because of the disturbance of particle networks in the polymer matrix. The piezoresistance effect becomes more prominent if the matrix material is compliant making these materials attractive for applications that require flexible force and displacement sensors such as e-textiles and biomechanical measurement devices. However, the exact mechanisms of piezoresistivity including the relationship between the matrix polymer, conductive particle, internal structure and the composite’s piezoresistance need to be better understood before it can be applied for such applications. The objective of this thesis is to report on the development of conductive polymer nanocomposites for use as flexible sensors and electrodes. Electrically conductive and piezoresistive nanocomposites were fabricated by a scalable melt compounding process. Particular attention was given to elucidating the role of matrix and filler materials, plastic deformation and porosity on the electrical conduction and piezoresistance. These effects were parametrically investigated through characterizing the morphology, electrical properties, rheological properties, and piezoresistivity of the polymer nanocomposites. The electrical and rheological behavior of the nanocomposites was modeled by the percolation-power law. Furthermore, a model was developed to describe the piezoresistance behavior during plastic deformation in relation to the stress and filler concentration.

My Ph.D. thesis is presented in two parts. In the first part, I describe the preparation of organic-inorganic hybrid micelles formed from poly(styrene-b-4-vinylpyridine) (PS-b-P4VP) block copolymers and CdSe quantum dots (QDs). Several distinct morphologies were observed including, spheres, finite-sized wormlike networks and clusters of hollow vesicles. A series of experiments were carried out to explore whether these hybrid colloids were thermodynamically stable or formed under kinetic control. Upon addition of 2-propanol (2-PrOH) to a chloroform solution containing a mixture of PS404-b-P4VP76 plus CdSe QDs (2-PrOH is a good solvent for P4VP block and a precipitant for PS block and QDs), uniform spherical micelles formed almost instantly, with a PS core and a thin P4VP corona to which the QDs were attached. Vigorous stirring of this solution for two days led to the formation of three-dimensional wormlike networks consisted of Y-junctions and cylindrical struts, terminated by bulbous spherical end-caps. Even more profound structural changes occurred when the solution was subjected to prolonged magnetic stirring (e.g. 1 month). ii In contrast, manipulating the chemical composition of the initial block copolymer could trigger a spontaneous structural transition from sphere to network of wormlike micelles over 2 h without the need of stirring. The second part of the thesis begins by describing a modular approach for preparing polyferrocenyldimethylsilane (PFS) block copolymers via a Cu-catalyzed alkyne/azide coupling reaction to covalently combine two homopolymers synthesized separately. This strategy opens the door to a broad library of novel functional PFS block copolymers, for example, poly(ferrocenyldimethylsilane-b-N-isopropyl acrylamide) (PFS-b-PNIPAM). In an attempt to expand our understanding of PFS block copolymer self-assembly in polar solvents, I investigated the self-assembly of a new polymer (PFS26-b-PNIPAM105) in alcohol solvents. When the block polymer was dissolved in methanol, ethanol and 2-propanol, it formed long fiber-like micelles with uniform width. I also showed that micelles of this polymer underwent seeded growth in methanol, leading to cylindrical micelles that were nearly mono- dispersed in length.

My Ph.D. thesis is presented in two parts. In the first part, I describe the preparation of organic-inorganic hybrid micelles formed from poly(styrene-b-4-vinylpyridine) (PS-b-P4VP) block copolymers and CdSe quantum dots (QDs). Several distinct morphologies were observed including, spheres, finite-sized wormlike networks and clusters of hollow vesicles. A series of experiments were carried out to explore whether these hybrid colloids were thermodynamically stable or formed under kinetic control. Upon addition of 2-propanol (2-PrOH) to a chloroform solution containing a mixture of PS404-b-P4VP76 plus CdSe QDs (2-PrOH is a good solvent for P4VP block and a precipitant for PS block and QDs), uniform spherical micelles formed almost instantly, with a PS core and a thin P4VP corona to which the QDs were attached. Vigorous stirring of this solution for two days led to the formation of three-dimensional wormlike networks consisted of Y-junctions and cylindrical struts, terminated by bulbous spherical end-caps. Even more profound structural changes occurred when the solution was subjected to prolonged magnetic stirring (e.g. 1 month). ii In contrast, manipulating the chemical composition of the initial block copolymer could trigger a spontaneous structural transition from sphere to network of wormlike micelles over 2 h without the need of stirring. The second part of the thesis begins by describing a modular approach for preparing polyferrocenyldimethylsilane (PFS) block copolymers via a Cu-catalyzed alkyne/azide coupling reaction to covalently combine two homopolymers synthesized separately. This strategy opens the door to a broad library of novel functional PFS block copolymers, for example, poly(ferrocenyldimethylsilane-b-N-isopropyl acrylamide) (PFS-b-PNIPAM). In an attempt to expand our understanding of PFS block copolymer self-assembly in polar solvents, I investigated the self-assembly of a new polymer (PFS26-b-PNIPAM105) in alcohol solvents. When the block polymer was dissolved in methanol, ethanol and 2-propanol, it formed long fiber-like micelles with uniform width. I also showed that micelles of this polymer underwent seeded growth in methanol, leading to cylindrical micelles that were nearly mono- dispersed in length.

Synthesis of nanocrystals is one of the most rapidly advancing areas of nanoscience, and today nanocrystals can be produced with impressive control over their composition, size, shape, polydispersity, and surface chemistry. As such, they are ideal building blocks for fabricating hierarchical architectures with tailorable functionality on every level of the hierarchy. Here an evaporation-driven, template-assisted nanocrystal assembly (ETNA) technique is developed, providing a novel and general approach to fabricating freestanding, 3D, functional architectures using diverse combinations of colloidal nanocrystal species and porous templates of arbitrary geometry. Colloidal PbS (photoluminescent) and CoFe2O4 (superparamagnetic) nanocrystals are template-assembled to fabricate freestanding nanorods and inverse opals, which retain the size-dependent properties of their constituent building blocks while replicating the geometry and preserving the functionality of the templates. Further multifunctionality is demonstrated through mixed-nanocrystal architectures which exhibit the aggregate functionality of their building blocks.

This study investigates the possibility of fabricating periodic cellular materials (PCMs) via the lost foam casting (LFC) process using aluminum alloy A356 and magnesium alloy AZ91. This approach combines the structural efficiency of PCM architectures with the processing advantages of near-net-shape LFC. An initial feasibility study fabricated corrugated A356 panels. This was followed by a study of casting variables such as pattern design, vacuum assistance, and alloying additions in order to improve the fillability of the small cross-section struts. Finally, integrated pyramidal sandwich panels having different relative densities were subjected to artificial aging treatments and subsequently tested in uniaxial compression. The A356 PCMs experienced a continuous increase after yielding while the AZ91 PCMs exhibited strut fracture after peak strength. The results showed the compressive yield strengths of this study are comparable with those previously reported PCMs produced by different fabrication methods.

Two novel hybrid materials for use in sandwich cores of structural materials are designed, manufactured and mechanically tested. Each material is a hybrid of metal and polymer foam. One set of hybrids is fabricated using an aluminium micro-truss filled with varying densities of polyurethane foam. Increases up to 120% in stiffness, 372% in strength, 740% in resilience and 106% in impact energy over the aluminium micro-truss are obtained from compression and impact testing. Furthermore, the stiffness of these hybrids can be tailored according to the density of the polyurethane foam. Another set of hybrids is fabricated using a rapid prototyped ABS polymer truss that is foamed and electroplated with nanocrystalline nickel. Increases up to 1525% in stiffness, 1165% in strength and 650% in energy absorption over the foamed ABS truss are obtained. Furthermore, the gain in strength, stiffness and energy absorption outweigh the gain in density in these hybrid materials.

A standard procedure for P3HT:PCBM bulk-heterojunction solar cells has been developed. Fabrication conditions, such as environment; solution concentration, thickness of active layer or post-treatment methods are systematically optimized. The best device performance is obtained by slow-drying spin-coated P3HT:PCBM (1:0.8) blend layer with DCB as solvent. C60:LiF composite films with up to 80% LiF concentration as hole blocking layer have been developed to significantly increase power conversion efficiencies of OPV devices. The short-circuit current density is greatly enhanced, without sacrificing open-circuit voltage and fill factor. Due to its superior oxygen diffusion blocking effect, the C60:LiF composite layer also can provide a more effective passivation film than a thin LiF layer, resulting in an impressive enhancement in air stability of devices.

Porous constructs of calcium polyphosphate (CPP) are under investigation as a substrate for tissue engineering of cartilage for repair of osteochondral defects. Previous studies have shown that CPP has the required features to satisfy these requirements. However, its degradation rate is lower than desired. This study investigated the effect of doping with MgCO3, MgCl2, K2CO3 or KCl at a molar ratio of M/Ca = 0.02 on sintering and in vitro degradation behavior of CPP. Doping with magnesium or potassium improved the tensile and compressive strengths of CPP at similar porosities. After 15 days of aging in phosphate buffer saline, the rate of tensile strength loss was faster for the doped CPP groups than undoped CPP. The chemical degradation rate of Mg-doped CPP groups was the fastest among CPP groups. The chemical degradation rate of K-doped CPP groups was slower than undoped CPP.

A scanning reference electrode technique (SRET) apparatus has been designed and commissioned to investigate the corrosion of friction stir spot welds (FSSW) made in AZ31 magnesium alloys. The operational parameters of the apparatus have been calibrated to give good spatial resolution. By combining the SRET data with material flow data and immersion test data it was found that the FSSW process caused the formation of distinct noble and active regimes within the weld area. The noble region was aligned with the stir zone (SZ) and was caused by a dynamically recrystallized grain structure which is void of dislocations / twins, and β Mg17Al12. Localized corrosion attack was observed in both SRET and immersion testing along the thermo-mechanically affected zone (TMAZ). The same effect was consistently observed with a flat versus concave shoulder tool, and dwell times of 1s and 4s.

The oxidation behavior of model binary and ternary alloys in supercritical water (SCW) was examined. Binary alloys contained 9 or 14 at% Cr. Ternary alloys had 1.5 at% of one of the following elements: Si, Al, V, Mn or Ti. Samples with different surface finishes were exposed to SCW for up to 500 hours. The oxidized samples were characterized using gravimetry, SEM, HRTEM and EDX. After exposure, a uniform double layer oxide with outwardly grown magnetite inwardly grown Fe-Cr mixed oxide was detected on most of samples. Alloys contains 14 at% Cr or/and 1.5 at% Si showed a mixed oxidation mode, where relatively thick double layer oxides coexisted with thin protective oxide. The coverage with the thin oxide increased with the increase of deformation and/or Cr and/or Si content. A 20 nm thick Si-enriched layer was detected at such alloy/oxide interfaces. This behavior is very similar to “third element effect”.